In a vehicle equipped with a fuel cell system, a hydrogen detector is generally provided to detect the occurrence of a leakage of hydrogen supplied as a fuel gas for fuel cells. An ECU (electronic control unit), for example, determines whether a hydrogen concentration detected by the hydrogen detector exceeds a preset threshold value. In response to the detected hydrogen concentration exceeding the preset threshold value, an alarm lamp may be lit to inform the user of the occurrence of a hydrogen leakage. One proposed technique sprays a test gas onto the hydrogen detector provided in the vehicle to check the condition of the hydrogen detector (see, for example, Japanese Patent Laid-Open No. 2006-329786 and No. 2004-93204).
The test gas sprayed onto the hydrogen detector to check the condition of the hydrogen detector typically has a higher concentration than the preset threshold value. Lighting of the alarm lamp informs the user of the normal functionality of the hydrogen detector.
In the case where multiple hydrogen detectors are installed in the vehicle, the respective hydrogen detectors may have different threshold values as criterion for determining the normal functionality or the abnormality. For example, one hydrogen detector may be installed in a fuel cell casing for storage of fuel cells, while another hydrogen detector may be installed in proximity to a hydrogen tank. The fuel cell casing has a limited space and accordingly has a high potential for ignition even at a significantly low concentration of hydrogen. A relatively small value should thus be set to the threshold value for the hydrogen detector installed in the fuel cell casing. The hydrogen detector installed in the proximity to the hydrogen tank is close to the hydrogen tank and thus naturally tends to detect a high hydrogen concentration. A relatively large value should thus be set to the threshold value for the hydrogen detector installed near to the hydrogen tank. In this manner, multiple different threshold values should be set according to the installation locations of the respective hydrogen detectors by taking into account the size of the space and the distance from a location with a potential hydrogen leakage.
In order to check the conditions of the multiple hydrogen detectors, multiple test gases having different gas concentrations should be provided according to the installation locations of the respective hydrogen detectors. This undesirably makes the checkup of the hydrogen detectors rather complicated. The test gas used for the checkup purpose typically has a fixed gas concentration. Providing the multiple test gases having different gas concentrations corresponding to the multiple different threshold values, however, undesirably consumes both the time and the cost.
This problem is not characteristic of the hydrogen detectors installed in the vehicle but may be commonly found in hydrogen detectors installed in any of various other locations, for example, inside a garage or inside a building. This problem is also not characteristic of the hydrogen detectors but may be commonly found in other gas detectors configured to detect various gases, for example, gasoline, methanol, carbon monoxide, and propane gas.